The important pathological changes in Alzheimer's disease are described with their morphology, biochemistry and selective topography. The most significant hypotheses concerning the origin of these changes, their interrelationship and link with mental deterioration are reviewed. Some of the characteristic findings, such as neurofibrillary tangles, senile plaques, granulo-vacuolar degeneration, amyloid angiopathy, death and regeneration of neurons, are analyzed. The main neurotransmitter defects are mentioned in brief and similarities and differences between normal aging and Alzheimer's disease emphasized.